In recent years, optical discs called “next-generation DVDs” have begun to appear in the market, such as Blu-ray discs and HD-DVDs (Digital Versatile Discs), for which blue-violet laser light is used in an optical disc apparatus. An optical disc apparatus that accommodates such an optical disc is also required to be compatible with legacy optical discs such as CDs (Compact Discs) and DVDs. Therefore, the optical pickup is required to have three laser light sources of different emission wavelengths and an actuator for driving one or two objective lenses.
As with conventional optical disc apparatuses for CDs/DVDs, there is a demand also for such optical disc apparatuses to increase the recording/reproduction speed and to further reduce the thickness thereof so that they can be installed in notebook PCs, or the like.
In order to realize a reduction in the thickness of an optical disc apparatus, it is necessary to reduce the thickness of the optical pickup. In order to increase the recording/reproduction speed, it is necessary to increase the driving power of the actuator and also to reduce the weights of the objective lens and the actuator, which form the moving section.
An optical disc apparatus using blue-violet laser light has a working distance (the distance from the objective lens to the optical disc surface; hereinafter referred to as the “WD”) as small as about 0.3 mm, whereas a conventional optical disc apparatus for CDs/DVDs has a WD as large as about 0.9 mm.
The center of gravity of the moving section will deviate substantially from the center about which the actuator is driven if one designs an objective lens actuator so as to accommodate all optical discs of substantially different WDs while taking into consideration that an actuator needs to have a sufficient range of motion in order to accommodate the surface deviation of an optical disc.
FIG. 10 is a cross-sectional view showing a moving section of an optical pickup, showing an actuator including a casing on which an objective lens is fixed. Since it is necessary to arrange an objective lens on the upper side of the moving section in order to accommodate Blu-ray discs with a smaller WD, the center of gravity 401 of the moving section deviates from the center 402 about which the moving section is driven.
One way to lower the center of gravity is to provide a counterweight under the moving section, but it will be difficult to drive the moving section at a high speed if the weight increases.
With such an actuator, driving the objective lens along the tracking driving direction 403 generates a force also in a direction shown as the rolling direction 404, thereby causing the objective lens to roll and thus tilting the objective lens with respect to the recording surface of the optical disc. The tilt of the objective lens occurring when the actuator is driven in the tracking direction as described above is called an AC tilt. As shown in FIG. 8, the amount of the AC tilt increases in proportion to the amount of displacement of the actuator in the tracking direction, and rapidly increases with the frequency of displacement. Patent Document 1 proposes a method for correcting such an AC tilt.
If a sensor for constantly detecting the AC tilt occurring is attached to the optical pickup, it is possible to correct the AC tilt according to the output of the sensor. However, in order to realize a thin optical pickup, it is difficult to reserve a space for installing the sensor therein.
Another possible way to correct the AC tilt is to correct the tilt based on the current supplied to the tracking coil when driving the actuator in the tracking direction.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2003-022552